Minecraft poor man's enderman farm?

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Okay, here's the deal, first off, I do NOT need a tutorial on how to build this, because I have an automatic one already, BUT here's the thing, I was killing a bunch of enderman in it and one teleported into the middle where I was standing, you can guess what happened next. So here's what I need, I need to put some kind of block to prevent them from coming in, but that I can still be able to press the kill button through the block, keep in mind I'm doing this in SURVIVAL mode.

All you need to do is put a roof in with the room with the button so there's only two blocks of height in the room. Endermen need a three block space to teleport in (by default) so a roof that is only two blocks high can keep them out.

You can also put half-slabs on the floor and make the roof 'three' (really 2.5) blocks high as well.

Endermen also take damage in water. Coat the floor with water so that there's nowhere to stand without being in the water and any Enderman that jumps in will retreat and switch back to non-hostile.

Otherwise in Vanilla Minecraft there's not a block that can prevent the teleport. I believe there are mods that add such a block though.